1. Field of the Invention
The present invention relates to a digital angular modulator for obtaining from an input analog or digital signal an angular-modulated signal, such as a frequency-modulated (FM modulated) signal, a frequency-shift keying (FSK modulated) signal, and a phase-shift keying (PSK modulated) signal.
2. Description of the Background Art
A digital FM modulator is disclosed in U.S. Pat. No. 5,091,705 to Hiramatsu et al. entitled "FM MODULATOR". Prior to the FM modulator, another type of digital FM modulator as shown in FIG. 1 was disclosed in Japanese patent laid-open publication No. 56-152359 (1981). The two types of FM modulator share an analog-to-digital (A/D) converter 5 and the main structure from an integrator 8 to digital-to-analog (D/A) converters 10 and a quadrature modulator 11. The chief difference between the two types is that although the Hiramatsu et al. FM modulator integrally performs the D/A conversion and the quadrature modulation, the FM modulator of '359 publication separately carries out the D/A conversion and the quadrature conversion.
In FIGS. 1A and 1B, an analog input signal (modulation signal) is converted into a digital signal by an A/D converter 5 at a sampling period Ts. The digital signal is fed through a digital waveform processor 6 to the integrator 8 which integrates the output of the digital waveform processor 6 to produce the phase component of the complex envelope of the modulation signal. The output of the integrator 8 is divided into quadrature (cosine and sine) components by a complex envelope generator 9. The cosine and sine components are D/A converted into analog signals by the D/A converters 10-1 and 10-2, and quadrature modulated by the quadrature modulator 11, thus generating an FM modulated signal. It is well known that an FM modulator can be structured by an integrator and a PM modulator (see, for example, page 331 of "COMMUNICATION SYSTEMS ENGINEERING", by J. G. Proakis, et al. Prentice Hall, 1994), and thus the quadrature modulator 11 functions as a PM modulator in this system. The more detailed structure and operation of the FM modulator will become apparent later in the description of embodiments.
The quadrature FM modulator has an advantage that it can achieve stable modulation index and produce an accurate FM signal without adjustment because it obtains the instantaneous phase of the modulation signal by the digital processing. In addition, the FM modulator is effective to obtain a quadrature phase-shift keying (QPSK modulated) signal with a single modulator by shifting the analog modulation signal or digital data by .pi./4.
The quadrature FM modulator as shown in FIGS. 1A and 1B, however, has the following problems:
(1) To reduce the sampling noise and quantization noise involved in the A/D conversion of the modulation signal by the A/D converter 5, the sampling frequency of the A/D converter 5 must be set higher to increase the bits of the quantization. PA1 (2) In converting the digital signals representing the complex envelope output from the complex envelope generator 9 to the analog signals by the D/A converters 10-1 and 10-2, the phase variation of the complex envelope increases with the modulation factor. To reduce the error of the phase variation, the sampling frequency of the A/D converter 5 must be increased so as to decrease the instantaneous phase variation per sampling period.